中野 博生

Abstract Recent advances in the orthogonal-dimer spin- Heisenberg antiferromagnet are reviewed. This model has been examined for more than 40 years since it was originally studied by Shastry and Sutherland. The discovery of SrCu (BO ) as a good candidate material for the system led to a considerable advancement in subsequent research. In the case when is larger than 1/2, however, the elucidation of the properties of the system was relatively delayed. In the ground state of general- systems, interesting quantum phase transitions occur not only for . In this article, the systematic behavior depending on which has been recently clarified by unbiased and numerical computational calculations is reviewed. The effects due to spin anisotropy as determined from experimental and theoretical results are also overviewed. We also mention recent developments for the system.

Abstract The spin- S Heisenberg antiferromagnet on the two-dimensional lattice is investigated for S = 1/2 and S = 1. We consider interaction at isolated dimers ( J _d ) and interaction bonds that form the bounce lattice ( J _b ). For J _d = J _b , the system is reduced to the maple-leaf-lattice antiferromagnet. We primarily conduct highly parallelized numerical diagonalization to examine the spin excitation gap above the ground state for various J _b / J _d cases. For S = 1/2, we report calculations for a 42-site cluster that has not been previously treated. The S = 1 case is examined for the first time for clusters up to 24 sites. Regardless of whether S = 1/2 or 1, we find that the system has a gapped nature for small J _d / J _b and becomes gapless at J _d / J _b ∼ 1.4. For S = 1, we also find that another gapped region appears between the gapless case at J _d / J _b ∼ 1.4 and the boundary of the exact-dimer phase.

The magnetization process of the S = 1 spin ladder with two anisotropies competing each other is investigated using the numerical diagonaization of finite-size clusters and the level spectroscopy analysis. As a result, it is found that the translational symmetry broken magnetization plateaux appear at 1/4 and 3/4 of the saturation magnetization. The phase diagrams with respect to the anisotropies at the 1/4, 1/2 and 3/4 magnetizations are obtained. The magnetization curves for several typical parameters are also presented.